This invention relates to the production of high titania slag from ilmenite and control of such a process.
Natural rutile, owing to its high titanium content and low levels of problem-causing impurities, has traditionally been the preferred feed stock for the production of titanium tetrachloride as an intermediate towards the production of titanium dioxide. Natural rutile is becoming scarcer and, consequently more costly, and the alternative method that uses ilmenite is becoming more favoured.
Ilmenite concentrates have a relatively low titanium content (usually about 50% titanium dioxide compared to about 96% in the case of rutile) and the high impurity content in the balance renders ilmenite generally unsuitable for direct chlorination to titanium tetrachloride as in the case of rutile. In consequence, ilmenite has been used as feed stock for the production of pigment by the sulphate process, which is becoming increasingly environmentally objectionable.
The alternative to the sulphate route, namely the chloride route, has a major problem associated therewith in that the direct chlorination of ilmenite results in a large quantity of ferric chloride being produced with an associated waste disposal problem. The chloride production of pigment is however preferred over the sulphate route because it requires less processing energy and yields a pigment of superior quality.
In general, manufacturers of the intermediate titanium tetrachloride are economically unable to process feed stocks containing less than 80% TiO2 and, in addition, impose stringent limits on some impurities, in particular calcium and magnesium. The latter two elements are undesirable in fluidised bed chlorinators because they form high boiling point chlorides, which tend to clog both the bed itself and gas ducting leading from the reactor. The usual specification imposed on calcium and magnesium is that the sum of the oxides of these two elements should not exceed 1.2% by mass. In some cases the limits are imposed as not greater than 0.2% calcium oxide and not greater than 1.0% magnesium oxide.
In consequence of the unsuitability of ilmenite for direct use in the chloride process, processes involving the thermal reduction of ilmenite to produce a titania rich slag as well as a process involving combined prereduction and chemical leaching procedures to form a synthetic rutile have been proposed and used.
Of these the thermal reduction approach yields a product having a lower titanium grade but it does have the advantage of producing iron in a directly recoverable state.
Conventional alternating current open-arc smelting of various types of ilmenite ores is currently being carried out. The feed materials are introduced into a conventional six-in-line open-arc smelter via multiple feed ports (typically more than 20 feed ports are used) most of which are situated near sidewalls of the furnace in order to protect the furnace sidewalls from refractory erosion. This type of feeding renders process control extremely difficult and, as a result, irrespective of sophisticated computerised control that can be applied to such furnaces, localised regions of overreduced slag are periodically produced which gives rise to a foaming slag. This is regarded as loss of control and corrective measures, which need to be taken, lower the thermal efficiency and availability of the furnace.
Ilmenite has also been smelted in a direct current (D.C.) transferred-arc plasma furnace, on test level and also on commercial level. Some of the problems experienced in the testing of the D.C. smelting of ilmenite included wear of the furnace lining, which are normally magnesite based, that results in unacceptably high product contamination. It is also difficult to establish and maintain a frozen lining to act as a barrier between the refractory lining and furnace charge.
One of the problems with the frozen lining is that it is very sensitive to fluctuations in the power density in the furnace, which may result in a decrease in the frozen lining thickness and a simultaneous release of contaminants into the furnace charge, or an increase in the frozen lining thickness that reduces the internal volume of the furnace and lowers its efficiency.
It is an object of this invention to provide a process for the thermal reduction of ilmenite in which the problems mentioned above is at least partly obviated.
In accordance with this invention there is provided a process for the reduction of ilmenite in a D.C. transferred arc furnace having a refractory lining and operating with a molten bath, one or more electrodes situated in the roof of the furnace acting as cathode, the molten bath acting as anode, and a frozen lining at least partly between the refractory lining and the molten bath; the process comprising feeding the ilmenite simultaneously with carbonaceous reductant, in the absence of fluxes, to the molten bath, withdrawing titania rich slag and pig iron from the furnace, and means to control the operation of the furnace; the control means including means to take temperature measurements of a furnace wall adjacent the frozen lining, means to estimate the thickness of the frozen lining as a function of the temperature in the furnace wall and means to control the amount of titanium dioxide produced in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.
There is also provided for the control means to include means to measure furnace gas flow rate, furnace gas temperature and furnace gas composition; means to measure furnace cooling water flow rate and furnace cooling water temperature, means to measure feed rates of the ilmenite and the carbonaceous reductant, means to measure the furnace electrical system variables including furnace power input and furnace resistance, and means to measure the ilmenite and carbonaceous reductant composition.
The invention further provides for means to estimate the frozen lining thickness and furnace hot face temperatures as a function of the furnace wall temperature measurements and furnace gas flow, furnace gas composition and furnace gas temperature measurements.
There is also provided for the means to control the amount of titanium dioxide produced in the furnace to include control over the rate of addition of carbonaceous reductant to the furnace, for the amount of titanium dioxide produced to increase with an increase in the rate of addition of carbonaceous reductant to the furnace to thereby urge the thickness of the frozen lining to increase; and for the amount of titanium dioxide produced to decrease with a decrease in the rate of addition of carbonaceous reductant to the furnace to thereby urge the thickness of the frozen lining to decrease.
There is also provided for means to calculating heat losses from the furnace as a function of the furnace gas flow, furnace gas composition and furnace gas temperature, the cooling water flow rate and cooling water temperatures; and for means to measure sensible heat changes of spray cooled roof panels, spray cooled off gas ducts, film cooled shell panels, air cooled hearth panels, hot gases and dust, and charge removed from the furnace.
The invention also provides for means to estimate a material balance of the furnace as a function of the estimated frozen lining thickness, the ilmenite and carbonaceous reductant feed rates, the ilmenite carbonaceous reductant, slag and pig iron composition measurements, and the furnace power input and furnace resistance measurements.
There is further provided for means to perform inventory control over the furnace using the material balance of the furnace.
The invention further provides for means to estimate a future titanium dioxide composition of the slag and carbon content of the pig iron as a function of the estimated frozen lining thickness and furnace wall temperatures, the calculated heat changes, the feed and product compositions and temperature measurements, the furnace gas composition, the furnace gas flow rate and furnace gas temperature measurements, the amount of slag and pig iron tapped from the furnace and the internal and sensible heat energy content of the tapped slag and pig iron, and power input to the furnace.
There is also provided for means to perform chemistry control of the furnace using the estimated material balance, the current and predicted slag titanium dioxide composition and pig iron carbon content, the ilmenite and carbonaceous reductant feed rate setpoints, the furnace power input, and the ilmenite and carbonaceous reductant composition measurements.
The invention further provides for means to perform start-up control of the furnace using the feed rate setpoints, the furnace power and resistance setpoints, the ilmenite and carbonaceous reductant composition measurements, and the calculated heat losses.
There is also provided for a process of error detection and validation to be conducted on all of the above measurements, for the process of error detection and validation to include analysis of the range of the measurements and the rate of change of the measurements to validate the measurements, and for invalid measurements to be labelled as bad quality, and also for further calculations using these invalid measurements to be labelled as bad quality, until such time as the error has been corrected.
The invention further provides for the furnace to be circular, for one or more of the electrodes to be hollow and to serve as a feed port for at least part of the ilmenite and the carbonaceous reductant, and for the ilmenite and the carbonaceous reductant to be fed to a central region of the furnace.
The invention also provides a method for controlling a frozen lining between a furnace lining and a molten bath in a D.C. transferred arc furnace used for the continuous reduction of ilmenite, the method comprising the steps of:
1. establishing the frozen lining,
2. measuring at least the temperature in a wall of the furnace adjacent the frozen lining,
3. estimating the thickness of the frozen lining as a function of the temperature in the wall, and
4. controlling the amount of titanium dioxide produced in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.
There is also provided for step 2 of the method to include measuring furnace gas flow rate, furnace gas temperature and furnace gas composition, furnace cooling water flow and temperature, ilmenite and carbonaceous reductant feed rates, furnace power input and furnace resistance, and ilmenite and carbonaceous reductant composition measurements.
The invention also provides for the method to include performing a process of error detection and validation on the measurements, the process of error detection and validation including the steps of
a) analysing the range of the measurements and the rate of change of the measurements,
b) validating the measurements,
c) labelling invalid measurements as bad quality, and
d) labelling further calculations using the invalid measurements as bad quality until such time as the error has been corrected.
The invention further provides for step 3 of the method to include a step of estimating the thickness of the frozen lining as a function of the temperature in the furnace wall and the furnace slag and the furnace gas temperature.